1. Field of the Invention
The present invention relates generally to an internally heated pyrolysis zone, and more specifically to such an apparatus that can be used in the vacuum deposition of parylene monomer.
2. Discussion of the Related Art
Conformal coatings having low dielectric constants and high melting temperatures are desirable for many applications, including the medical, electronics and automotive industries. For example, the components of an automobile that are located beneath the hood (e.g., the carburetor) are exposed to various corrosive gases as well as temperatures in excess of 200.degree. C. for extended periods of time.
Conventionally, layers of parylene polymers have been used as conformal coatings since some of these layers are known to have dielectric constants of about 2.3 and melting temperatures of about 540.degree. C. Parylene polymers are poly-p-xylylenes which may be prepared with a parylene dimer. Parylene polymers generally have a structure: ##STR1## wherein X is typically a hydrogen atom or a halogen atom. The most common forms of parylene dimers include the following: ##STR2##
Typically, parylene polymer layers are formed by a vapor deposition method that is performed using a vacuum apparatus 10 as shown in FIG. 1, such as disclosed in U.S. Pat. No. 5,538,758 which is herein incorporated by reference. Apparatus 10 includes a vaporization zone 12, a pyrolysis zone 14, a post-pyrolysis zone 16 and a deposition chamber 18.
In this method, generally referred to as the Gorham process, the parylene dimer is vaporized in vaporization zone 12 and the dimer bond is then cleaved by pyrolysis in pyrolysis zone 14 to form parylene monomer having the structure: ##STR3##
The parylene monomer passes through post-pyrolysis zone 16 which removes a substantial amount of undesired chemical species. The parylene monomer is then deposited onto the surface of a substrate 19 disposed within deposition chamber 18. Polymerization of the deposited parylene monomer occurs spontaneously to form the parylene polymer layer coating on the surface of substrate 19.
FIG. 2 shows a pyrolysis zone 14 which includes a tube 20 with one or more electric tube heaters 22 disposed along the outer surface 24 of wall 26 of tube 20. Heaters 22 increase the temperature of tube 20 to heat the parylene dimer disposed within the interior of tube 20, resulting in cleavage of the parylene dimer bond and formation of the parylene monomer. Due to the arrangement of heaters 22, parylene dimer disposed adjacent the inner surface 28 of wall 26 of tube 20 is maintained at a higher temperature than parylene dimer disposed adjacent the centerline 29 of tube 20. However, since the gas flow rate through tube 20 is generally highest along centerline 29 and lowest along inner surface 28, tube 20 may not be effective in heating the parylene dimer flowing adjacent centerline 29 to a temperature high enough to pyrolyze the parylene dimer and form parylene monomer. At the same time, overpyrolysis of parylene dimer adjacent inner surface 28 can occur, causing a carbonaceous residue to build up on surface 28. Typically, greater than about 1 weight percent of the vaporized parylene dimer results in carbonaceous buildup on inner surface 28.
Thus, it is desirable within the art to provide an apparatus that offers improved efficiency in pyrolyzing a parylene dimer to form parylene monomer which can subsequently be deposited on a surface to yield a parylene polymer layer.